1. Field of the Invention
The present invention relates to a centrifugal fan.
2. Description of the Related Art
A centrifugal fan for generating an airflow through the use of an impeller is known in the art. For example, Japanese Patent Application Publication No. 2009-537735 discloses a conventional centrifugal blower including three impellers and an electric motor. In this blower, the motor, the impellers, the inlet port and the outlet port are arranged in a coaxial relationship.
The inlet port is arranged at one end portion of the substantially cylindrical blower and the outlet port is arranged at the other end thereof. Air is sent from the inlet port to the outlet port through three stages arranged along an axis. The air is accelerated in the respective stages and is forwarded from the impellers toward the downstream side through a radial outer region. However, with this structure, it is difficult to reduce the size of the blower. In medical devices such as, for example, in an artificial respirator and an expectoration assist device, it is desirable to reduce the size of a blower from the viewpoint of portability.
Static pressure is one of the indices of a performance level of a centrifugal fan. In order to increase the static pressure, it is possible to increase the revolution number of a motor or to lengthen the flow path within a housing. In case of increasing the revolution number of a motor, it is however necessary to take countermeasures against heat generation. If an attempt is made to merely lengthen the flow path in the blower of Japanese Patent Application Publication No. 2009-537735, the axial dimension of the blower grows quite larger. In medical devices such as, for example, an artificial respirator and an expectoration assist device, it is required to increase the static pressure while also reducing the size of a blower.